Stretching wire generates current

Researchers have developed a new type of electric power generator able to produce ac current through cyclical stretching and releasing of zinc oxide wires. The new “flexible charge pump” generator is the fourth generation of devices designed to produce electrical current by using the piezoelectric properties of zinc oxide structures to harvest mechanical energy from the environment.

“The flexible charge pump offers yet another option for converting mechanical energy into electrical energy,” says Zhong Lin Wang, director of the Center for Nanostructure Characterization at the Georgia Institute of Technology. The generator can produce an oscillating output voltage of up to 45 millivolts. To boost the current produced, arrays of the flexible charge pumps could be constructed and connected in series. Multiple layers of the generators could also be stacked to form modules that could be implanted in the human body to power blood pressure sensors or other devices.

When the modules are mechanically stretched and released, the zinc oxide material generates a piezoelectric potential that alternately builds up and then is released. The wires are encapsulated in a flexible plastic substrate with two bonded ends; a Schottky barrier controls the alternating flow of electrons, and the piezoelectric potential is the driving force of the charge pump.

To measure generated electric energy in tests, researchers subjected the substrate and attached zinc oxide wires to periodic mechanical bending created by a motor-driven mechanical arm. Bending induces tensile strain, which creates a piezoelectric potential field along the laterally packaged wires. That in turn drives a flow of electrons into an external circuit, creating the alternating charge and discharge cycle – and corresponding current flow. Wang envisions a family of small-scale generators enabling development of a new class of self-powered wireless sensing systems that could gather, store, and transmit information without an external power source.